Ultrasonic imaging technology has become an important tool for examining the internal structure of living organisms. In the diagnosis of various medical conditions, ultrasonic imaging is often useful to examine soft tissues within the body to show the structural detail of internal tissues and fluid flow. An important application of ultrasonic imaging is in the detection and identification of various internal structural abnormalities, such as cysts, tumors, abcesses, mineral deposits, blood vessel obstructions, and anatomical defects without physically penetrating the skin.
Ultrasonic images are formed by producing very short pulses of ultrasound using an electro-acoustic transducer, sending the pulses through the body, and measuring the properties (e.g., amplitude and phase) of the echoes from tissues within the body. Focused ultrasound pulses, referred to as "ultrasound beams", are targeted to specific tissue regions of interest in the body. Typically, an ultrasound beam is focused at small lateral and depth intervals within the body to improve spatial resolution. Echoes are received by the ultrasound transducer and processed to generate an image of the tissue or object in a region of interest. The resulting image is usually referred to as a B-scan image.
Measuring and imaging tissue motion and blood flow within a living body is typically done using the Doppler principle, in which a transmitted burst of ultrasound pulses at a specific frequency is reflected from the moving tissue, thereby changing the frequency of the reflected ultrasonic wave in accordance with the velocity and direction of tissue motion. The Doppler frequency shift of reflected signals with respect to the transmitted signals is proportional to the velocity of tissue motion. The mean frequency shift and its amplitude at each Doppler sampling location may be detected and displayed on a video device to provide graphic images of moving tissue or fluid flow within a living body.
The detection and identification of tumors, in particular, are often accomplished during real-time interactive ultrasonic imaging of internal tissues. Manual compression of internal tissues by applying pressure on the ultrasonic transducer probe through the skin may provide useful information about the elasticity or softness of various components of internal organs as the operator observes how the various components respond to applied manual pressure (changing shape, sliding, rolling, etc.).
The use of mechanical vibration at an audio frequency may reduce the variability of the magnitude and rate of compression while preserving freehand scanning in a light-weight hand-held probe. The vibration is continuous, and the vibrational source is controlled by well-defined control settings--frequency, output power, waveform, etc. This method combines separate ultrasound and audio transducers to send vibrations into a living body and ultrasonically detect the induced tissue vibrations using the Doppler principle. A transmitted burst of ultrasound at a specific frequency is reflected from moving tissue, changing the frequency of the reflected ultrasound in accordance with the velocity and direction of tissue vibration. The Doppler frequency shift of reflected signals with respect to the transmitted signals is proportional to the velocity of tissue motion. Whenever this frequency exceeds a low-frequency noise threshold, its amplitude or variance may be detected and displayed on a video display device to provide graphic images of moving tissue structure within a living body on the basis of its vibrational properties.
Present ultrasonic tissue motion imaging techniques include frequency-shift color Doppler imaging (CDI), power color Doppler imaging, and variance color Doppler imaging of tissue motion, as well as cross-correlation ultrasound estimation of displacements and mean velocities (such as color velocity imaging, or CVI, developed by Philips Corporation, and elastographic imaging techniques as developed by the University of Texas). These present known methods of ultrasound tissue motion imaging provide relatively limited information regarding the physical properties and direction of motion in a complex medium or a living body. For example, present CDI techniques primarily provide the frequency-shift that is dependent on both the velocity of tissue motion or fluid flow and the Doppler angle between the ultrasound beam and the direction of motion or flow. They may also provide the amplitude of Doppler signals that is dependent on the number and reflectivity of moving tissue reflectors. However, CDI techniques do not provide any information on the mechanical properties of living tissues. Cross-correlation techniques detect and display a limited range of velocities of motion or flow. Although cross-correlation methods can provide information on tissue elasticity and compressibility when viewed during application of an externally-applied mechanical stress, the algorithms involved are complex and computationally intensive, often involving acquisition of radio frequency (RF) data, thus requiring increased processing time and computer resources. Furthermore, because of the uncertainty in cross-correlation estimates of velocity and errors introduced by lateral decorrelation, the signal-to-noise ratio and spatial resolution of these methods have typically been limited. A related technique, vibrational Doppler imaging (VDI), provides elastographic information based on induced vibration at a given frequency and power color Doppler imaging of induced tissue motion. VDI provides improved spatial and contrast resolution as well as improved reproducibility of compression in a hand-held probe. Because VDI images contain a mixture of both acoustic reflectivity, tissue elasticity, and vibrational resonance information, they may be diagnostically ambiguous, depending on the chosen vibrational frequency.
The present invention, a vibrational resonance ultrasonic Doppler spectrometer, detects and differentiates embedded structures and tissues on the basis of vibrational resonance properties that may be affected by elasticity, size and mass of structural subunits, vibrational damping properties, etc. These mechanical properties may not be detectable or visible with most prior ultrasonic imaging methods. In a heterogeneous compressible medium, such as living tissue, vibrational resonance is observable in domains of locally more echogenic material (acoustic scatterers) which are elastically coupled through less echogenic tissues to other nearby domains as shown schematically in FIG. 7. The more echogenic scatterers matter only because they are the foci that provide the detectable Doppler echoes upon which vibrational resonance spectrometry and imaging are based. In addition, the vibrational resonance ultrasonic Doppler imager displays the spatial distribution of these properties in a two-dimensional imaging slice of the medium or tissue. Because vibrational resonance ultrasonic Doppler spectrometry and imaging both utilize an entire range of vibrational frequencies, much greater information on the mechanical properties of various compressible media or soft tissues is acquired with which to differentiate them on the basis of a vibrational resonance "signature". This provides potentially improved sensitivity and specificity over previous ultrasound imaging techniques, including VDI, and may be particularly useful in detecting and differentiating tumors in soft organs such as the breast, prostate, and liver.